The long-term goal of the proposed research is to clarify the molecular mechanisms underlying the detection and discrimination of chemicals through contact chemosensation in the fruit fly, Drosophila melanogaster. Contact chemosensation allows flies to distinguish sweet from bitter molecules, as well as nonvolatile pheromones. Insect gustatory organs express a diversity of candidate molecular detectors. These include gustatory receptors (GRs), TRP channels, ionotropic receptors (IRs) and odorant binding proteins (OBPs), the latter of which promote the detection of chemicals by receptor proteins. However, the functions of most of these candidate gustatory receptors and binding proteins are unknown, or are understood poorly. We propose experiments to dissect the mechanisms underlying contact chemosensation in flies using a multidisciplinary approach that includes electrophysiology, behavior, genetics, and cell biological approaches. During the last few years, the concept that GRs are required broadly for sensing sugars and bitter-tasting compounds has been confirmed. However, the biochemical functions of GRs are unclear. Aim 1 is to test the hypothesis that GRs are tastant-activated cation channels. Aim 2 addresses one of the longstanding questions in taste sensation- the nature of sour receptors. These receptors are thought to be cation channels, and many candidates have been suggested. We propose experiments to dissect whether an IR is required for the responses to most sour tastants, which cells require the IR, and investigate the contributions of two other IRs that are expressed primarily in gustatory receptor neurons. Aim 3 is devoted to characterizing the roles for odorant binding proteins (OBPs) in contact chemosensation. OBPs are known primarily for promoting the detection of certain olfactory stimuli. We found that some OBPs are highly enriched in taste organs up to 800- fold over other tissues. Experiments are proposed to test the hypothesis that gustatory OBPs promote behaviors mediated by nonvolatile pheromones. Finally, the goal of our last aim is to dissect the molecular, cellular and biological basis for selective taste plasticity. We present preliminary data indicating that flies exposed to camphor reduce their repulsion specifically to camphor and not other bitter tastants, and this taste plasticity results from a reduction in the level of a TRP channel. Aim 4 is to distinguish between different possible models underlying this taste plasticity, and to address the biological basis for adaptation to some, but not all, unappealing tastants. An additional long-term goal of this research is to apply the findings to the control of insect pests that spread disease.